A transfer device according to the intermediate transfer method is known that is installed in an image forming apparatus adopting the electrophotography method in which toner images are transferred in such a manner as to be superimposed sequentially from a plurality of image bearing members onto an intermediate transfer belt in the primary transfer and then the toner image is transferred from the intermediate transfer belt onto a paper sheet in the secondary transfer.
Among the image forming apparatus are ones that have operational modes such as monochromatic transfer mode, full-color transfer mode, and standby mode in which no image forming is performed. The intermediate transfer belt is in contact with only an image bearing member for black in the monochromatic transfer mode, is in contact with all the image bearing members in the full-color transfer mode, and is separate from all the image bearing members in the standby mode.
For example, transition between the intermediate transfer belt's contact with and separation from the image bearing members is achieved by causing a plurality of primary transfer rollers respectively facing each of the plurality of image bearing members across the intermediate transfer belt to be displaced in directions for the transition between the contact and the separation (for example, refer to Patent Literature 1).
The conventional transfer device as described in the Patent Literature 1 that is provided with a link member for monochrome to cause a primary transfer roller for monochrome that corresponds to a black toner image to be displaced in the contact-separation directions, a link member for color to cause primary transfer rollers for color that correspond to toner images of the three primary colors (cyan, magenta and yellow) to be displaced in the contact-separation directions, and a transfer member moving mechanism that includes a cam to move the link member for monochrome and the link member for color respectively and a driving source to turn the cam. The cam is formed in an eccentric cam made up of a cam section for monochrome and a cam section for color that are to be turned around an identical cam shaft, both sections being displaced from one another in a direction of the cam shaft and fixed to each other. The respective cam sections for monochrome and for color are in contact with one end of respective link members for monochrome and for color at a portion in each peripheral surface of the respective cam sections.
The peripheral surface of each cam section is made up of a circular arc part (hereinafter referred to as a pressed-contact generating face) at which a distance (radius) from a center of rotation of the cam becomes greatest, and a part at which the radius becomes shorter (hereinafter referred to as a release generating face) than the former. While the link member is in contact with the pressed-contact generating face of the cam section, the link member moves along its longitudinal direction in a direction to go farther from the cam shaft, and then the primary transfer roller is displaced in a direction to approach the image bearing member, thereby causing the intermediate transfer belt to be pressed against the image bearing member. On the other hand, while the link member is in contact with the release generating face of the cam section, the link member moves along its longitudinal direction in a direction to come closer to the cam shaft, and then the primary transfer roller is displaced in a direction to separate from the image bearing member, thereby causing the intermediate transfer belt to be released from the image bearing member. In this manner, the intermediate transfer belt is caused to be pressed against and be released from the image bearing member for monochrome by the link member for monochrome moving along its longitudinal direction; and the intermediate transfer belt is caused to be pressed against and be released from the image bearing members for color by the link member for color moving along its longitudinal direction.
In the monochromatic transfer mode, the link member for monochrome is in contact with the pressed-contact generating face of the cam section for monochrome while the link member for color is kept in a state of being in contact with the release generating face of the cam section for color, thereby causing the primary transfer roller for monochrome to be displaced in a direction to approach the image bearing member for monochrome. In the full-color transfer mode, the link members for monochrome and for color are both kept in a state of being in contact with corresponding pressed-contact generating faces of the cam sections, thereby causing all the primary transfer rollers for monochrome and for color to be displaced in directions for the pressed contact. In the standby mode, the link members for monochrome and for color are both kept in a state of being in contact with corresponding release generating faces of the cam sections, thereby causing all the primary transfer rollers for monochrome and for color to be displaced in directions for the separation.
In each of the modes, the driving source to turn the cam is stopped. Therefore, there is a risk that sliding occurs between the peripheral surface of the cam section and the link member, and that thereby the cam unintentionally turns and then the link member comes off the pressed-contact generating face to the release generating face of the cam section. Due to the occurrence of such coming off of the link member, the amount of movement of the link member, namely the amount of displacement of the primary transfer roller, becomes unstable; therefore, a state of the pressed contact of the intermediate transfer belt with the image bearing member varies, from which a problem arises that a harmful effect is exerted on the transfer of the toner image.